On 7/23/19 1:04 PM, Johannes Weiner wrote: > CCing Jens for bio layer stuff > > On Tue, Jul 23, 2019 at 10:02:26AM +1000, Dave Chinner wrote: >> Even better: If this memstall and "refault" check is needed to >> account for bio submission blocking, then page cache iteration is >> the wrong place to be doing this check. It should be done entirely >> in the bio code when adding pages to the bio because we'll only ever >> be doing page cache read IO on page cache misses. i.e. this isn't >> dependent on adding a new page to the LRU or not - if we add a new >> page then we are going to be doing IO and so this does not require >> magic pixie dust at the page cache iteration level > > That could work. I had it at the page cache level because that's > logically where the refault occurs. But PG_workingset encodes > everything we need from the page cache layer and is available where > the actual stall occurs, so we should be able to push it down. > >> e.g. bio_add_page_memstall() can do the working set check and then >> set a flag on the bio to say it contains a memstall page. Then on >> submission of the bio the memstall condition can be cleared. > > A separate bio_add_page_memstall() would have all the problems you > pointed out with the original patch: it's magic, people will get it > wrong, and it'll be hard to verify and notice regressions. > > How about just doing it in __bio_add_page()? PG_workingset is not > overloaded - when we see it set, we can generally and unconditionally > flag the bio as containing userspace workingset pages. > > At submission time, in conjunction with the IO direction, we can > clearly tell whether we are reloading userspace workingset data, > i.e. stalling on memory. > > This? Not vehemently opposed to it, even if it sucks having to test page flags in the hot path. Maybe even do: if (!bio_flagged(bio, BIO_WORKINGSET) && PageWorkingset(page)) bio_set_flag(bio, BIO_WORKINGSET); to at least avoid it for the (common?) case where multiple pages are marked as workingset. -- Jens Axboe
